1. Technical Field of the Invention
The present invention relates to a hot-rolled steel plate manufacturing apparatus and method for pressing a long material such as a continuously cast slab in a plate thickness direction, and to a plate thickness press apparatus and method used for the apparatus and the method.
2. Description of the Related Art
1. In hot rolling of a thin plate such as a hot-rolled steel plate, a slab 20 is typically rolled by a roughing mill 7 so as to obtain an intermediate thickness (a rolled material in this state is referred to as a sheet bar), and it is thereafter rolled by a finishing mill 3 so as to have a thickness of a final product. Here, as to the dimension of the slab 20, the dimension of a heating furnace 13 for heating the slab 20 is an upper limit. As a result, the steel whose amount corresponds to one steel converter is usually divided into ten or more slabs 20. It is to be noted that the slab is referred to as a hot slab or simply referred to as a material according to needs.
A sheet bar 20A outputted from the roughing mill 7 has a defect of shape called a tongue or a fishtail necessarily produced at front and rear ends in greater or lesser degrees, similarly as in rolling of a regular plate. Incidentally, the xe2x80x9ctonguexe2x80x9d means a defect of shape that a central portion at the end in the plate width direction protrudes in the tongue-like form. The xe2x80x9cfishtailxe2x80x9d means a defect of shape that the both edges at the end in the plate width direction protrude in the fishtail-like form. Since both the tongue and the fishtail have the width narrower than that of a normal portion, they are apt to be easily deformed.
If these defects of shape are left as they stand, the deformation is further advanced by the finishing mill 3 in the next step, which may cause rolling trouble. The defects of shape are, therefore, cut and removed at the stage of the sheet bar 20A. The product yield is reduced as the cut and removed portion (which will be referred to as a xe2x80x9ccropxe2x80x9d hereinafter) becomes longer.
The finishing mill 3 is a continuous rolling mill generally composed of several stands and performs rolling to a steel tape having a thin thickness with tensile force applied thereto. A portion distanced from a front end of the hot-rolled steel plate by approximately 100 meters which has been subjected to finishing rolling is, however, rolled with no tensile force acting thereon until the front end reaches coilers 5a and 5b. Further, in this period, since traveling of the front end becomes unstable due to lifting and the like caused by collision with a carrier roll or a wind blast, rolling must be carried out by reducing a rolling speed to approximately half of that in the steady state (after reaching the coilers) in general.
Further, a shape of a rear end is also degraded because the tensile force becomes zero after it moves out from a final stand of the finishing mill 3. Such a non-steady portion is typically inferior to a steady portion in material and shape because cooling becomes uneven due to reduction in a temperature in conveyance or a defect of shape. Since rolling trouble caused due to such a defect of material and shape or meandering involved by the defect of shape lowers the capacity utilization ratio, this can be a serious adverse factor for reduction in the yield.
For improvement in the yield in finishing rolling, a method for connecting multiple sheet bars to each other to perform finishing rolling has been developed. For example, Japanese Patent Application Laid-open No. 84109-1992 proposes a method by which the front end of a sheet bar is sequentially coupled to the rear end of a preceding sheet bar so that finishing rolling is continuously performed to multiple sheet bars.
With this prior art technique, since rolling similar to that in the steady state is possible with respect to the coupled front and rear ends, the yield of the front and rear ends (non-steady portions) can be improved. Further, as to the front end portion, rolling can be performed at the same rolling speed as that in the steady state (after reaching the coilers), thereby improving the rolling efficiency. Furthermore, since a plurality of sheet bars are connected to each other to be rolled, the rolling efficiency can be more improved as compared with intermittent rolling.
Besides, there are also proposed other methods for manufacturing a long sheet bar such as a method for coupling multiple slabs or a method for directly rolling a continuously cast slab. As the method for coupling multiple slabs, Japanese Patent Application Laid-open No. 106403-1982 proposes a method by which a front end of a slab is sequentially coupled to a rear end of a preceding slab and the coupled multiple slabs are continuously rolled into a sheet bar by a planetary mill group.
Moreover, Japanese Patent Application Laid-open No. 92103-1984 proposes a method by which a slab whose amount corresponds to one steel converter is turned into a sheet bar by a rolling mill having a large thickness reduction amount to be wound around a coil as it is, and the coil of this sheet bar is then rewound to perform finishing rolling. Similarly, Japanese Patent Application Laid-open No. 85305-1984 proposes a method by which a slab cast by a special continuous casting machine (which is referred to as a rotary caster) at a high speed is turned into a sheet bar by rolling, it is once taken up to be wound in a rewinding machine and finishing rolling is thereafter carried out.
According to these conventional methods, crop cutting only at the front and rear ends of the long sheet bar can suffice, and the crop does not occur for each slab. The yield can be thus improved. Additionally, according to these methods, finishing rolling can obtain advantages similar to those in the method by which multiple sheet bars are connected to each other to perform finishing rolling.
These prior art techniques, however, have the following problems.
At first, in the method disclosed in Japanese Patent Application Laid-open No. 89109-1992, a part having a defective shape at the front and rear ends of the sheet bar must be cut off in order to connect the multiple sheet bars. The problem of reduction in the yield due to generation of the crop, therefore, remains. Further, a connected portion of the sheet bars has the strength lower than that of any other portion, and fracture at the connected portion in the middle of finishing rolling may force stoppage of the line. Furthermore, since the sheet bars are actually connected by welding, the structure of the connected portion becomes rough and large, which may possibly lead to generation of a defect of material or of a surface crack.
Moreover, in the method for coupling multiple slabs disclosed in Japanese Patent Application Laid-open No. 106403-1982, since the plate thickness of the slab to be coupled is large, it is difficult to completely couple the slabs in a short period of time. In addition, even if they are coupled in a short period of time, a hydrostatic pressure component as well as tensile stress may act on the coupled portion to cause peeling of the coupled surface when finishing rolling is conducted with a large thickness reduction amount. Therefore, the reduction amount must be decreased, and the efficiency of roughing rolling lowers.
Additionally, in the method for directly rolling a continuously cast slab disclosed in Japanese Patent Applications Laid-open Nos. 92103-1984 and 85305-1984, there is a problem that the efficiency of rolling is reduced due to limitation in a casting speed. According to the latter patent application, it is determined that the casting speed of 10 mpm is possible as the casting ability (weight per unit time), but there is actually no example reporting casting at such a high speed has achieved success in light of the operation and the quality.
As similar to the conventional techniques, in the method for directly rolling a continuously cast slab, a rolling speed of the roughing rolling mill at an initial stage is decreased to approximately several m/min at most owing to restriction in the casting speed. When this speed is converted into a number of roll revolutions of the rolling mill, it becomes approximately 1 rpm (1 minxe2x88x921), which is rolling at a very low speed. As a result, a roll of the rolling mill comes into contact with a material having a high temperature of approximately 1200xc2x0 C. for a long period of time (several seconds). Therefore, surface cracking, deformation or seizure of the roll may disadvantageously occur. Therefore, aside from a small facility, the above method can not be realized in a facility which has a large scale for manufacturing a hot-rolled steel plate and the like and deals with a high-temperature material.
Additionally, if the method for winding the sheet bar around the coil is applied to a regular hot rolling factory for a thin plate, a size of the coil for sheet bars is assumed to be comparable to several product coils, which results in a huge coil whose weight is approximately 100 tons. As a result, the coiling facility such as a winding machine and the like can not help becoming large, which is a problem in light of the facility cost, a space in the factory and others.
2. In a hot-rolled steel plate manufacturing line (a hot strip mill) or a continuously cast and directly rolled steel plate manufacturing line, a plate thickness press apparatus for pressing and forging the slab in the plate thickness direction is provided between the heating furnace or the continuous casting machine and the roughing mill. As a result, the hot slab is pressed in the plate thickness direction by the plate thickness press apparatus so as to obtain a target plate thickness size, and it is subsequently roughing-rolled. Then, finishing rolling is applied to the slab. Such a plate thickness press apparatus and method are disclosed in, for example, Japanese Patent Application Laid-open No. 238401-1986 or 274305-1990.
In plate thickness pressing disclosed in Japanese Patent Application Laid-open No 238401-1986, however, plate thickness pressing is carried out after the slab is subjected to width reduction rolling, and the slab subjected to width reduction rolling has such an advantage as that the width hardly returns to an original value at the time of plate thickness pressing. This plate thickness pressing, however, does not specify a type of width reduction which is applied to the front and rear ends of the material. When the slab is simply subjected to width reduction rolling from the front end to the rear end and subsequently pressed along the plate thickness direction, the front end and the rear end of the slab transform into flare shapes as shown in FIG. 1(b), and these parts must be cut and removed in the post-step, thereby reducing the yield. Further, in the former plate thickness pressing, even if rolling in the width direction is carried out before plate thickness pressing, the high reduction ratio at the time of plate thickness pressing causes a fluctuation in the width of the stationary portion after plate thickness pressing irrespective of execution/omission of width rolling. Furthermore, a lap (two-fold) or a bulge such as shown in FIG. 1(c) is generated on the cross section at the front end corner portion in the longitudinal direction irrespective of execution/omission of width rolling.
On the other hand, in plate thickness pressing disclosed in Japanese Patent Application Laid-open No. 274305-1990, although plate thickness pressing is conducted after the slab is subjected to width reduction pressing, the reduction speed of plate width and plate thickness pressing is very much slower than that of rolling. Therefore, reduction in a temperature of the slab is large, and plate thickness pressing is not therefore practical.
Moreover, according to the conventional plate thickness pressing method for the hot slab, when the hot slab is pressed in the plate thickness direction by a die 6 as shown in FIGS. 2(a) to 2(d), the slab 20 is fed by a fixed feed amount f, and a following portion is subjected to plate thickness pressing by the die 6. This is further fed by a fixed feed amount f. This process is repeated. A press working surface of the die 6 is constituted by a parallel portion 6a and a tapered portion 6b. A one-stage taper is usually adopted. The die 6 having a taper angle xcex8 of 10xc2x0 to 15xc2x0 (the taper angle is typically 12xc2x0) is often used. When the slab 20 is subjected to plate thickness pressing by the pressing apparatus having such a die 6, there occur forward elongation and backward elongation that the slab 20 is elongated forwards and backwards in the longitudinal direction as shown in FIG. 2(b). In the slab having such forward elongation and backward elongation generated, widthwise extension occurs in the non-steady portion in the flare form, and width distribution occurs at the steady portion in the wave-like form due to intermittent processes.
In the conventional plate thickness pressing method, if the taper angle xcex8 is small, a widthwise extension quantity becomes large, and a load also tends to become large. In this case, the width distribution dW (=Wxe2x80x2xe2x88x92W) is small. Although suppression of the widthwise extension and of increase in the load is possible by enlarging the taper angle, a slip may disadvantageously occur to the material during pressing depending on increase in the width distribution and pressing conditions.
There is also means for effecting transformation dispersion by using a tandem plate thickness pressing machine having multiple dies to reduce the plate thickness in plural stages, but this leads to a complicated and expensive apparatus.
Additionally, in the prior art, in case of reducing the thickness of the slab, the slab was caused to pass between rolls of a horizontal mill and subjected to thickness reduction by rolling. However, since a thickness which can be reduced by one rolling is small, multiple horizontal mills were provided at plural stages, or reverse rolling for reciprocating one horizontal mill was used. Such a method, however, results in a large-scale facility, a large installation space and large reduction in temperature of the slab which is being rolled. Thus, thickness reduction press for reducing the thickness by pressing at a stroke has been developed. However, when the thickness is largely reduced at a stroke, the reduced volume expands in the widthwise direction of the slab, thereby requiring forming in the widthwise direction.
Japanese Patent Application Laid-open No. 235002-1986 discloses an apparatus which performs width forming by providing vertical rolls on the downstream side of a thickness reduction press. FIG. 3 is a view showing a basic structure of this apparatus. In this drawing, there are provided a thickness reduction press 21 for sandwiching the slab 20 to press vertically arranged dies 21a by a cylinder 21b, and an edger 22 which is arranged on the downstream side of the thickness reduction press 21, provides rolls 22a with a flange on both widthwise ends of the slab 20 in the vertical direction and presses the rolls 22a with a flange in the widthwise direction. A regular rolling mill 23 is provided on the downstream side of the edger 22. With this arrangement, the slab 20 is pressed by the thickness reduction press 21 to reduce the thickness and widthwise extension is then corrected by the edger 22. Since the widthwise pressing by the edger 22 generates a dog bone that the width edge portion becomes thick, the dog bone is corrected by the rolling mill 23 arranged on the downstream side of the edger 22.
In the hot rolling facility having plate thickness reduction pressing apparatus provided therein, since an amount of reduction obtained by pressing is larger than that obtained by a rolling mill, a forming material such as a slab flows in the four directions as the thickness of the forming material is reduced. Paying notice to a width end portion in particular, this portion is formed into a corrugated shape larger than that obtained by rolling. When this end portion is rolled by a rolling mill group provided on the downstream side in this state, this corrugated shape is further amplified. In the prior art, therefore, as disclosed in the above patent application, an edger constituted by a vertical roll is arranged on the downstream side of the plate thickness reduction press to correct the corrugated shape of the width end portion. However, when an amount of reduction obtained by the thickness reduction press increases, the corrugated shape generated at the width end portion becomes also large. Even if the capability of the edger is increased, its function exceeds the limit, and the sufficient correction is impossible.
3. Further, the hot-rolled steel plate is generally manufactured from a hot slab by rolling and the like. In recent years, there has been developed a technique for applying forging to the hot slab by a die having a tapered portion in a material input side. As an example, there is a technique for forging from the plate thickness direction as similar to plate thickness pressing.
FIG. 4 shows a side elevation of a part of a general die used for forging the hot slab. It is to be noted that the die is composed of a pair of dies vertically arranged so as to sandwich the hot slab. FIG. 4, however, shows only the die on one side for the sake of convenience.
A side surface of the die 6 is a main processing surface constituted by a parallel portion 6a parallel to a material feeding direction, a tapered portion 6b inclined toward the input side with respect to the moving side of a material, and a transition area 6c between the parallel portion 6a and the tapered portion 6b. Here, an angle xcex8 of the tapered portion 6b relative to the parallel portion 6a is generally 10 to 15 degrees.
Description will now be given as to a method for forging the hot slab by using such a die with reference to FIGS. 5(a) to (c). By this method, the die is moved in the vertical direction with respect to the material longitudinal direction (moving direction), i.e., a gap in the plate thickness direction of the material is periodically changed to then forge the material.
At first, the die 6 is arranged in the vertical direction with respect to the moving direction of the hot slab 20 as shown in FIG. 5(a), and the hot slab 20 is then fed toward the die 6 (the n-th pass, before pressing). Then, the hot slab 20 is pressed by the die 6 as shown in FIG. 5(b) (the n-th pass, during pressing). Subsequently, the die 6 is departed from the hot slab 20 as shown in FIG. 5(c), and the hot slab 20 is then fed by a predetermined amount (the (n+1)th pass, before pressing). It is to be noted that reference character H denotes a plate thickness of the hot slab 20 before pressing and h designates a plate thickness of the hot slab 20 after pressing in FIG. 5(b).
Further, besides the method illustrated in FIGS. 5, there is also a method by which the material is continuously moved in the longitudinal direction during pressing as similar to a flying type material and the die moves in the longitudinal direction in order to reduce a relative velocity to the material.
In the above-described forging method, however, a slip may occur during pressing, this is an operational problem. That is, in case of pressing the hot slab 20 from the state before pressing as shown in FIG. 6(A), there occurs a phenomenon such that the hot slab 20 moves backwards without being pressed as shown in FIG. 6(B). When a slip is generated, the hot slab 20 is not subjected to a process for a specified feed amount. A number of times of pressing must be, therefore, increased, which lowers the operation efficiency. Furthermore, a trace of the slip remains on the surface of the hot slab, which may deteriorate the surface quality of a product.
Japanese Utility Model Application Laid-open No. 5201-1993 discloses a pressing die which forms a groove, a protrusion or a bore on its surface coming into contact with the side surface of the slab and increases the friction coefficient to decrease a slip. In case of this utility model, however, the cost for processing the die is high or a frequency of replacement of the die is increased because of unavailability of the die due to abrasion of a worn groove. Moreover, since the groove or the protrusion on the die surface is transferred onto the surface of the material, this can readily cause a trace when forging the material in the plate thickness direction in particular.
Japanese Patent Application Laid-open No. 122706-1997 discloses a slip detection method for sizing press, by which a slip is detected from a press load or a feed amount of a carrier roll and restarts carriage of a material so as to obtain a specified feed amount when slip occurs. However, when forging a material from the plate thickness direction, the present invention has a problem that any damage to the material surface can not be avoided.
Further, as shown in FIGS. 5(a) to 5(c), in the conventional plate thickness press forging, the gap of the die 6 in a direction (namely, the plate thickness direction of the material) orthogonal to the material longitudinal direction (moving direction) is periodically changed while feeding the hot slab 20, thereby forging the plate thickness of the hot slab 20 to the plate thickness of the product. However, the hot slab 20 of, e.g., the flying type may continuously move in the longitudinal direction even during pressing, and the die 1 may move in the longitudinal direction in order to decrease the relative speed with respect to the hot slab 20. When the die 6 is used to press the hot slab 20, the hot slab 20 elongates toward the upstream end side (die input side) and the downstream end side (die output side) in the longitudinal direction as shown in FIG. 5(b). Quantities of elongation of the material at the both ends are referred to as a backward elongation amount RW and a forward elongation amount FW, respectively.
In the conventional method, in order to reduce the load and uniform transformation in connection with sizing press, a lubricant is supplied to the entire surface of the die from the tapered portion 6b to the parallel portion 6a so that the friction coefficient of the die 6 with respect to the hot slab 20 can be reduced and the load can be decreased.
In the prior art method, however, a slip occurs between the die 6 and the hot slab 20, and hence the material can not be efficiently pressed. Further, reducing the friction coefficient lowers the forward elongation amount FW, and a number of times of pressing is increased to decrease the production efficiency.
Furthermore, although the above-described conventional method can be used to perform plate thickness pressing with a large reduction amount so that the plate thickness distortion across the plate width of the material becomes not less than 0.5, the excessive load is applied to the rolling mill at the time of plate thickness pressing. For example, according to provisional calculations by the present inventors in case of forging a soft steel slab with the plate thickness of 250 mm (or 256 mm) to 100 mm, the excessive load of approximately 5 ton is applied to the rolling mill in terms of a load (width load) per unit width (1 mm). When this is applied to the hot-rolled slab to perform conversion, the load of approximately 5000 ton is generated. Therefore, a very large load is applied on the press rolling mill. When the press rolling mill is used under such an excessive load, a frequency of occurrence of faults of the press rolling mill becomes high, thereby reducing the duration of life.
1. The present invention intends to solve the above-described various problems. That is, it is a first object of the present invention to provide a method and an apparatus for manufacturing a hot-rolled steel plate by plate thickness pressing capable of manufacturing a long sheet bar without joining sheet bars or slabs.
To achieve the first object, according to a preferred first apparatus embodiment of the present invention, there is provided an apparatus for manufacturing a hot-rolled steel plate by plate thickness pressing, comprising: a rough processing facility for performing a thickness reduction process to a hot slab cast by, for example, a continuous casting facility in order to obtain a sheet bar; a finishing mill group for rolling the sheet bar obtained by the rough processing facility to acquire a hot-rolled steel plate having a predetermined plate thickness; and a coiler for winding the hot-rolled steel plate, these members being arranged in the mentioned order, wherein the rough processing facility includes forging means using a pair of dies each of which includes an inclined portion on an input side and a flat portion on an output side as at least a part of the thickness reduction processing means, and width reducing means is provided on the upstream side of the thickness reduction forging means.
Further, according to a preferred first method embodiment of the present invention, there is provided a method for manufacturing a hot-rolled steel plate by plate thickness pressing, comprising: a rough processing step for performing a thickness reduction process to a continuously cast slab having a plate thickness H to obtain a sheet bar; a finishing rolling processing step for rolling the sheet bar to obtain a hot-rolled steel plate having a predetermined plate thickness; and a winding step for winding the hot-rolled steel plate after cooling, wherein the rough processing step at least partly includes a plate thickness press processing step by which a pair of dies each of which includes an inclined portion on an input side and a flat portion on an output side and a plate thickness reduction ratio r is not less than 30%, and width reduction whose amount is not less than a width reduction amount determined by the following expression is applied to a material before the plate thickness press processing:
Width reduction quantity=fn(r, H)
The present invention presses a continuously cast slab in the plate thickness direction in place of performing rolling as a preliminary stage of roughing rolling. In this case, the plate thickness direction reduction ratio r is determined as not more than 0.3 in view of a generation ratio of internal defects such as a casting defect.
Subsequently a pair of vertical dies 6 each of which has a tapered portion 6b on an input side and a parallel portion 6a on an output side shown in FIG. 4 are used to perform the plate thickness pressing process. The tapered portion 6b is provided on the input side of the die 6 so as not to generate a step on the surface of a material at the end of the die 6. A material which comes into contact with the tapered portion 6b on the input side of the die has a reduction ratio r which continuously varies. This ratio is not less than 0.3 in the parallel portion 6a and becomes zero (r=0) in a non-contact portion. A trouble such as cracks on the surface due to generation of a step can be, therefore, avoided.
When the thickness of the material is reduced by the plate thickness pressing process, the reduction strain is distributed in the plate thickness direction of the material. The distribution becomes large in the plate width central portion where the plane strain can be observed, whilst the distribution is small in the plate end portion where the plane strain causing the widthwise deformation can be observed. Accordingly, evaluating the internal quality improvement effect by using a maximum value of the reduction strain distribution, the internal quality improvement effect is small at the plate end portion.
Therefore, reduction in the widthwise direction is carried out before the plate thickness pressing process, and a large plate thickness called a dog bone is formed at the plate end portion. Moreover, the plate thickness press processing is effected after increasing the plate thickness of the plate end portion. As a result, the reduction strain at the plate end portion can be increased to impart the internal quality improvement effect equivalent to that at the plate central portion.
Additionally, according to a preferred second method embodiment of the present invention, there is provided a method for manufacturing a hot-rolled steel plate by plate thickness pressing, wherein when a pair of dies each of which includes an inclined portion on an input side and a parallel portion on an outlet side are used to perform a plate thickness pressing process with a reduction ratio in a plate thickness direction of not less than 30% with respect to a continuously cast slab, a contact length L of the parallel portion of the die in a longitudinal direction falls within a range of 0.2 to 0.4 fold of the plate thickness of the slab on the inlet side at a front end of the slab, and continuous roughing rolling and subsequent finishing rolling are applied to the slab which has been subjected to the plate thickness press process, thereby obtaining a hot-rolled steel plate.
In the present invention, the continuously cast slab is pressed in the plate thickness direction instead of being subjected to rolling as a preliminary stage of roughing rolling. The reduction ratio of the plate thickness pressing is determined to be not less than 30% in view of a generation ratio of internal defects such as a casting defect. When the reduction ratio is determined to be not less than 30% in this manner, the generation ratio of internal defects can be decreased to 0.01% or lower.
As similar to the rolling process, the plate thickness pressing process causes the plate thickness central portion to protrude forwards from the both sides (generation of a bulge 28) or cave at an end portion of the material or, in particular, at the front end so that the outer surfaces overlap each other at the end portion (generation of a lap 27). The thus deformed portion must be cut and removed as a crop at a stage of a sheet bar after roughing rolling. In particular, as shown in FIG. 16(a), when the lap 27 is generated at the front end of the hot slab 20, this lap may cause a folded plate. The lap must be, therefore, completely removed.
The present inventors have eagerly studied about deformation of the hot slab at the front end and discovered that the deformation behavior of the front end varies depending on the plate thickness pressing process conditions. First of all, as a tendency as a whole, when the tapered portion 6b of the die comes into contact with the front end of the slab, the generation ratio of the lap 27 shown in FIG. 16(a) increases. When the parallel portion 6a of the die is brought into contact with the front end of the slab, both the lap 27 and the bulge 28 may occur as shown in FIG. 16(c).
As a result of the study, the present inventors have found that both a size of the lap 27 (length in the slab longitudinal direction) and a size of the bulge 28 can be adjusted by using a length L (which will be referred to as a xe2x80x9ccontact length Lxe2x80x9d hereinafter) of the front end of the slab which comes into contact with the parallel portion 6a of the die shown in FIG. 15. That is, as shown in FIG. 17, the lap 27 is readily generated in an area in which the contact length L is short. The generation frequency and the size of the lap 27 are decreased as the contact length L becomes long. On the contrary, the generation frequency and the size of the bulge 28 are increased as the contact length L becomes long. Therefore, by appropriately setting the contact length L, the generation frequencies of the lap 27 and the bulge 28 can be decreased to a low level. In addition, sizes of these non-steady deformation portions (length in a pass line direction) can be decreased.
Moreover, as a result of the strenuous study, the present inventors have unveiled that deformation of the front end of the slab largely depends on the plate thickness H of the hot slab 20 as well as the contact length L. Based on such information, the present inventors have completed the method according to the present invention, by which the contact length L and the plate thickness H are used to estimate a size of deformation at the front end of the slab (the lap 27 and the bulge 28).
FIG. 17 shows its result. In FIG. 17, a horizontal axis shows a ratio L/H of the contact length and the plate thickness, and a vertical axis illustrates a lap length L1 and a bulge length L2. FIG. 17 is a characteristic diagram showing a result of examining the influence of the contact length L and the plate thickness H on the lap length L1 and the bulge length L2. In the figure, a white triangle indicates generation of the lap 27, while a white square indicates generation of the bulge 28. Further, in the figure, a curve E corresponds to a characteristic line obtained by integrating areas in which the bulge 27 frequently occurs by the least squares method, and a curve F corresponds to a characteristic line obtained by integrating areas in which the lap 27 frequently occurs by the least squares method.
As apparent from FIG. 17, the dimension L1 of the lap 27 becomes long as the ratio L/H of the contact length L to the plate thickness H becomes smaller. On the contrary, the dimension L2 of the bulge 28 becomes long as the ratio L/H becomes large. In an intermediate area, although the lap 27 or the bulge 28 is generated, it is considered that this generation is caused due to irregularities in the temperature distribution.
When a range in which the generation frequencies of both the lap 27 and the bulge 28 lowers in the intermediate area is obtained from FIG. 17, the ratio L/H is not less than 0.2 and not more than 0.4 in that range. Based on this, the manufacturing method according to the present invention controls the plate thickness press processing of the front end of the slab in such a manner that the ratio L/H falls within the range of 0.2 to 0.4.
Further, if the ratio L/H is zero, i.e., if the front end of the slab 20 does not abut with the parallel portion 6a of the die but comes into contact with the tapered portion 6b, the generation frequency of the lap 27 is increased. In the actual operation, if the front end of the slab comes into contact with the inclined portion of the die, the hot slab 20 slips similarly as in the case of a nipping defect in the rolling process. This is not preferable because the pressing operation does not smoothly proceeds. As in the method according to the present invention, setting the ratio L/H in the range of 0.2 to 0.4 in light of the working property can obtain preferable results.
Further, in the present invention, since deformation of the front end of the slab can be controlled by pressing conditions, an excellent shape can be expected by roughing rolling. In general, the shape of the front end of the slab after rolling largely varies due to a temperature distribution of the slab, and the lap 27 occurs when a corner portion of the slab is excessively heated. On the contrary, when a surface temperature of the slab is lowered, generation of the bulge 28 can not be avoided. Accordingly, in the present invention, if the corner portion of the slab 20 is overheated, the contact length L is set longer to suppress generation of the lap 27 and minimize the lap size L1. On the other hand, when the surface temperature of the slab 20 is lowered, the contact length L is set shorter to suppress generation of the bulge 28 and minimize the bulge size L2.
Moreover, according to the present invention defined in a preferred third method embodiment, there is provided a method for manufacturing a hot-rolled steel plate by plate thickness pressing, wherein a pressing process with a reduction ratio of not less than 0.5 is applied to a continuously cast slab in a plate thickness direction by using a pair of dies each of which includes an inclined portion on an input side and a flat portion on an output side, pressing process conditions at this time are set in a range satisfying the following inequality represented by a contact length L of the inclined portion of the die and a material in a longitudinal direction, a feed quantity f, a plate width W before processing, a volume V to be processed by the parallel portion of the die, a plate width on the output side h, and a reduction strain xcex5, roughing rolling is continuously applied to the slab after pressing process, and finishing rolling is subsequently applied to the same to obtain a hot-rolled steel plate:
xcex5L/W less than Axe2x80x83xe2x80x83(1)
Vxcex5/(Wfh) less than Bxe2x80x83xe2x80x83(2)
where A and B are constants.
The present invention performs pressing to a continuously cast slab in the plate width direction instead of carrying out rolling as a preliminary stage of roughing rolling. In this case, the reduction ratio is determined to be not less than 0.5 in light of a generation ratio of internal defects such as a casting defect. As will be described later, it is desirable that the generation ratio of internal defects is set to 0.001% or lower in order to obtain the high quality. In the present invention, setting the reduction ratio to not less than 0.5 suppresses the generation ratio of internal defects to 0.001% or lower.
Although a pair of dies each of which has an inclined portion on an input side and a flat portion on an output side are then used to conduct pressing process. The inclined portion is provided on the input side of the die in order to prevent a step from being formed on the material at an end of the die. At the portion which has come into contact with the inclined portion of the die on the input side, the reduction ratio continuously changes from 0.5 or above in the flat portion to 0 in the non-contact portion, and a trouble such as a crack on the surface due to generation of a step can be hence avoided.
In the meanwhile, since the plate width of a material is increased by the pressing process, it is desirable to suppress its increasing amount as much as possible. As a result of earnestly examining factors influencing an increasing amount of the plate width, it was found that an aspect ratio of the material coming into contact with the inclined portion of the die, i.e., a ratio L/W of the contact length L in the longitudinal direction and the plate width W largely influences. It was discovered that an increasing amount of the plate width can be substantially adjusted by a product of this ratio L/W and the reduction strain xcex5 as will be described later. Consequently, setting the value xcex5L/W to a fixed value A or lower can suffice suppression of an increasing amount of the plate width to a predetermined value. When representing this by a formula, the above-described expression (1) is obtained.
As to the plate width in the longitudinal direction, it was unveiled that it slightly fluctuates due to a difference in a position where the material is brought into contact with the die. As a result of examining factors influencing this fluctuation of the plate width, it was found that the fluctuation relates to the processing status obtained from the flat portion of the die. It was consequently discovered that the fluctuation of the plate width is in proportion to the reduction strain obtained by only the flat portion and the overall reduction strain.
The processing strain obtained by only the flat portion can be estimated by a processing amount of a portion processed by the flat portion and the plate width h after processing. This processing amount can be expressed as a mean value using a ratio of a volume V and an area of the portion processed by the flat portion. Since an area of a portion processed by the flat portion is a product of the plate width W and a feed amount f, a processing amount of the portion processed by the flat portion can be expressed as V/(Wf).
As a result, the processing strain caused by only the flat portion is V/(Wf)/h or V/(Wfh). It was discovered that a fluctuation amount of the plate width can be substantially adjusted-by a product Vxcex5/(Wfh) of the ratio V/(Wfh) and the reduction strain xcex5, as will be described later. After all, setting the value Vxcex5/(Wfh) to a fixed value B or lower can suffice suppression of a fluctuation amount of the plate width to a predetermined value. When this is expressed by a formula, the above-described expression (2) is obtained.
2. It is a second object of the present invention to provide a plate width pressing apparatus and method capable of: (1) effectively preventing a flare from being produced at front and rear ends, preventing a steady portion width distribution, and effectively preventing a lap (two-fold) at a front end corner portion of a material; (2) minimizing a width distribution dw and suppressing increase in a load during pressing even if the material is pressed with a high reduction amount; and (3) modifying extension of a slab in a widthwise direction even if pressing with large reduction in thickness is used.
When the slab 20 shown in FIG. 1(a) is subjected to plate thickness pressing, an intermittent process by which the thickness is reduced in accordance with each fixed segment is carried out. Therefore, front and rear ends 20a of the slab are deformed in the flare shape as shown in FIG. 1(b). Further, a bulge or a lap (two-fold) is formed at the width central portion in the longitudinal cross section of the slab front end depending on pressing conditions. Prevention for such deformation is possible to some degree by adjusting the pressing conditions. However, the lap is formed at a corner portion of the front and rear ends as shown in the right-hand side of FIG. 1(c) irrespective of pressing conditions, and the lap must be cut and removed in the post-step.
As a countermeasure, the present inventors have eagerly studied about a deformation generation mechanism in a non-steady portion and consequently completed the present invention described below.
That is, to achieve the second object, according to the present invention defined in a preferred fourth method embodiment, there is provided a plate thickness pressing method for pressing a substantially rectangular material in a widthwise direction to adjust the width before performing plate thickness pressing to the substantially rectangular material in the plate thickness direction by using a die having a main processing surface consisting of at least an inclined portion on an input side and a parallel portion following the inclined portion with respect to the substantially rectangular material, wherein at least one of a front end and a rear end of the substantially rectangular material is pre-formed.
Further, according to the present invention defined in a preferred fifth method embodiment, in such a case, a non-steady width change quantity xcex94W and a non-steady length xcex94L produced in at least one of the front end and the rear end of the material by plate thickness pressing may be predicted by using the following expressions and the front end of the substantially rectangular material may be previously formed based on this prediction:
xcex94WH=f1(W,xcex5,Ldt), xcex94WT=f2(W,xcex5,Ldt)
xcex94LH=g1(W,h,Ldt), xcex94LT=g2(W,H,Ldt)
where, xcex94WH represents a predicted non-steady width change amount generated at the front end in a rectangular material moving direction by plate thickness pressing; xcex94WT, a predicted non-steady width change amount generated at the rear end in the rectangular material moving direction by plate thickness pressing; xcex94LH, a predicted non-steady length generated at the front end in the rectangular material moving direction by plate thickness pressing; xcex94LT, a predicted non-steady length generated at the rear end in the rectangular material moving direction by plate thickness pressing; H, a plate thickness of the substantially rectangular material on a press input side; h, a plate thickness of the substantially rectangular material on a press output side; xcex5(=log(H/h)), a plate thickness strain; Ldt, a contact length of the material and the press die in the longitudinal direction; and W, a plate thickness of the substantially rectangular material.
Additionally, according to a preferred sixth method embodiment of the present invention, pre-forming may be previously effected to provide a distribution to the plate width of the steady portion of the substantially rectangular material.
Further, according to a preferred seventh method embodiment of the present invention, a steady portion plate width distribution amount dW generated due to plate thickness pressing and its pitch dL may be predicted by using the following expressions and pre-forming may be performed to provide a distribution to the plate width of the substantially rectangular material steady portion based on this prediction. At this time, in the expressions dW=F(V, W, h, f, xcex5) and dL=G(H, h, f), H represents a plate thickness of the substantially rectangular material on the press input side; h, a plate thickness of the substantially rectangular material on the press output side; xcex5(=log(H/h)), a plate thickness strain; W, a plate width of the substantially rectangular material; f, a feed amount of the substantially rectangular material at the time of plate thickness pressing; and V, a reduction volume of the parallel portion of the die.
Moreover, according to a preferred eighth method embodiment of the present invention, the front end and the rear end of the substantially rectangular material may be previously formed in advance and pre-forming may be conducted to provide a distribution of the plate width of the steady portion of the substantially rectangular material.
Furthermore, according to a preferred ninth method embodiment of the present invention, a non-steady width change amount xcex94W and a non-steady length xcex94L generated in at least one of the front end and the rear end of the substantially rectangular material by the plate thickness pressing, a width distribution dW of the steady portion and its pitch dL may be predicted by using the following expressions, the front end and the rear end of the substantially rectangular material are pre-formed based on the prediction, and pre-forming may be performed to provide a plate width distribution of the substantially rectangular material steady portion:
where, xcex94WH=f1(W,xcex5,Ldt), xcex94WT=f2(W,xcex5,Ldt)
xcex94LH=g1(W,h,Ldt), xcex94LT=g2(W,H,Ldt)
dW=F(V,W,h,f,xcex5)
dL=G(H,h,f), and
xcex94WH represents a predicted non-steady width change amount generated at the front end in the rectangular material moving direction by plate width pressing; xcex94WT, a predicted non-steady width change amount generated at the rear end in the rectangular material moving direction by plate thickness pressing; xcex94LH, a predicted non-steady length generated at the front end in the rectangular material moving direction by plate thickness pressing; xcex94LT, a predicted non-steady length generated at the rear end in the rectangular material moving direction by plate thickness pressing; H, a plate thickness of the substantially rectangular material on the press input side; h, a plate thickness of the substantially rectangular material on the press output side; xcex5(=log(H/h)), a plate thickness strain; W, a plate width of the substantially rectangular material; f, a feed amount of the substantially rectangular material at the time of plate thickness pressing; V, a reduction volume of the parallel portion of the die; Ldt, a contact length of the substantially rectangular material and the press die in the longitudinal direction; H, a plate thickness on the material input side; and h, a plate thickness on the material output side.
According to preferred tenth and eleventh method embodiments of the present invention, the above-described width adjustment can be performed by a vertical rolling mill capable of changing an opening during processing. In this case, it is preferable to use a caliber roll.
According to a preferred twelfth method embodiment of the present invention, the above-described width adjustment can be carried out by a widthwise pressing machine which can be tandem with the plate thickness press. In this case, plate thickness forming and plate width forming can be sequentially performed.
According to the present invention defined in a preferred second apparatus embodiment, there is provided a plate thickness press apparatus comprising: a die having a main processing surface consisting of at least an inclined portion on an input side and a parallel portion following the inclined portion with respect to a substantially rectangular material; means for feeding the substantially rectangular material to the die; a plate thickness pressing device for driving the die to press in a plate thickness direction of the substantially rectangular material; and a vertical rolling mill which is provided on the pass line upstream side away from the plate thickness pressing device and can change an opening during processing.
Further, according to the present invention defined in a preferred third apparatus embodiment, there is provided a plate thickness press apparatus comprising: a die having a main processing surface consisting of at least an inclined portion on an input side and a parallel portion following the inclined portion with respect to the substantially rectangular material; means for feeding the substantially rectangular material to the die; a plate thickness pressing device for driving the die to press in a plate thickness direction of the substantially rectangular material; and a widthwise direction pressing device which is provided on a pass line upstream side away from the plate thickness pressing device and arranged at a possible where it can be tandem with the plate thickness pressing device.
Moreover, according to the present invention defined in a preferred thirteenth method embodiment, there is provided a plate thickness pressing method for performing cast and reduction in thickness while sequentially feeding a plate thickness of a substantially rectangular hot slab in a longitudinal direction, comprising: a main processing step for reducing a plate thickness H of the hot slab before pressing to a plate thickness h after pressing by a die having a main processing surface consisting of at least an input side tapered portion and a parallel portion; and a sub processing step for performing thickness reduction pressing in the plate width direction to a portion which is to be pressed by a transition portion corresponding to a boundary between the tapered portion and the parallel portion of the die having the main processing surface and a portion in the vicinity of the former portion before the main processing step.
Incidentally, according to a preferred fourteenth method embodiment of the present invention, assuming that a feed amount of the material is f and a material backward elongation amount at the time of pressing is BW in the sub processing step, it is preferable to press in the plate thickness direction a portion which is positioned on the upstream side away from the portion to be pressed by the transition portion by a distance determined by the following expression:
(0.9 to 1.1)xc3x97f+(fxe2x88x92BW)xc3x97n
where n is a positive integer.
Furthermore, according to a preferred fifteenth method embodiment of the invention, assuming that a feed amount of the material is f, the portion to be subjected to thickness reduction pressing in the sub processing step is a portion positioned on the upstream side away from the transition portion by a distance of (0.9 to 1.1)xc3x97f, and it is preferable to alternately perform the sub process and the main process.
In addition, according to a preferred sixteenth method embodiment of the present invention, assuming that a ratio of a thickness reduction amount by the sub process to a thickness reduction amount by the main process is r, it is preferable to set the thickness reduction amount by the sub process to be equal to or above (Hxe2x88x92h)xc3x97r(rxe2x89xa70.025).
Further, according to a preferred seventeenth method embodiment of the present invention, assuming that a ratio of a thickness reduction amount by the sub process to a thickness reduction amount by the main process is r, it is desirable that the sub process is started when the thickness reduction amount by the main process exceeds (Hxe2x88x92h)xc3x97(1xe2x88x92r). Furthermore, according to a preferred eighteenth method embodiment of the present invention, the main process and the sub process are simultaneously executed by using the same die. As a result, a number of dies can be reduced.
Moreover, to achieve the second object, according to the present invention defined in a preferred nineteenth method embodiment of the invention, a thickness of a slab is reduced by a thickness reduction press, and a width of the same is reduced by a width reduction press after releasing the thickness reduction press.
The thickness reduction press is used to reduce the thickness of the slab, and the width reduction press is then used to reduce the width of the slab. Since the width reduction press can increase the reduction capability, correction is enabled even if corrugated expansion deformation is large in the widthwise direction. Additionally, by operating the width reduction press when reduction is not carried out by the thickness reduction press, capacities of power sources of the both presses can be equal to a capacity of the thickness reduction press which is larger than that of the other press.
In addition, according to the present invention defined in a preferred fourth apparatus embodiment, there are comprised: a thickness reduction press for reducing a thickness of a slab; a width reduction press which is provided on the downstream side of the thickness reduction press and reduces a width of the slab; and a controller for operating the width reduction press when the thickness reduction press is released.
The thickness reduction press is first used to press the slab in order to reduce the thickness of the slab. A volume of the slab flows in four directions due to this thickness reduction, and corrugated expansion deformation is generated in the widthwise direction. The deformed portion is straightened and pressed by the thickness reduction press so as to obtain a predetermined width. The controller alternately operates the thickness reduction press and the width reduction press in such a manner that the both presses are not operated at the same time. Thus, capacities of power sources of the both presses can be reduced.
According to the present invention defined in a preferred fifth apparatus embodiment, a width measuring instrument for measuring a slab width is provided on the downstream side of the width reduction press, and the controller adjusts an opening of the width reduction press so that a measured value of the width measuring instrument becomes a predetermined value.
Although the controller sets an opening indicating a gap between dies of the width reduction press in order to control the width reduction press, the set value is constantly corrected based on the measured value of the width of the slab subjected to width reduction so as to obtain a predetermined slab width. The width of the slab expands beyond the gap between the dies when being pressed. Since this expansion amount varies depending on a temperature or a substance of the slab, a width of the slab before slab thickness reduction, a thickness reduction amount and others, such an opening as that a predetermined slab width can be obtained is predicted based on these conditions and the slab width measured value, and a direction is given to the width reduction press. In case of performing such prediction, the controller uses a learning calculation function for learning and predicting the relationship between the previous prediction and the measured value.
3. Moreover, it is a third object of the present invention to provide a plate thickness pressing method capable of: (1) preventing a slip from occurring at the time of pressing by forging a contact start surface between a hot slab and a die as a transition area between a tapered portion and a parallel portion and a part of the parallel portion without a need of a special forming process; (2) assuring a desired forward elongation amount in forging of a hot slab by using a die having a main processing surface consisting of a tapered portion on an input side and a substantially parallel portion such as a plate thickness press, reducing a generation frequency of slips between the die and a material, and decreasing a load applied to a press rolling mill.
To achieve the third object, according to the present invention defined in a preferred twentieth method embodiment, there is provided a hot slab manufacturing method for forging a hot slab by using a die having a main processing surface consisting of a tapered portion inclined in an input side direction with respect to a moving direction of the hot slab and a parallel portion which follows the tapered portion and is parallel to the moving direction, wherein a contact start surface of the hot slab and the die is a transition area between the tapered portion and the parallel portion and a part of the parallel portion.
Further, according to the present invention defined in a preferred twenty-first method embodiment, it is preferable to apply a lubricant on at least the contact surface relative to the hot slab in the main processing surface of the die.
This is based on the fact that use of the lubricant is very effective for reducing the load because a slip does not occur even if the friction coefficient is lowered in case of abutting from the parallel portion of the die. Here, as the lubricant, any kind of material can be used as long as it is a hot lubricant which acts to lower the friction coefficient, such as a mixture of a mineral oil (grease) and a solid lubricant, e.g., black lead, molybdenum disulfide or graphite, or solo use of the mineral oil. As to a position on which the lubricant is applied, although the lubricant is applied on at least the contact surface relative to the hot slab in the main processing surface of the die, the lubricant may be applied on a part of the die along the longitudinal direction and/or the widthwise direction or on the entire surface. Incidentally, changing the friction coefficient by processing a groove and the like on the surface of the die is not desirable since the surface of the die is transferred onto a material, which may cause a scratch.
In addition, as a method for applying the lubricant to the tapered portion of the die for example, a material is forged and a gap of the die is opened once. The lubricant is then sprayed toward the tapered portion of the die from the material input side direction by a nozzle while moving the material by a specified amount for forging of the next pass. On the other hand, the lubricant is similarly applied to the parallel portion of the die from the material output side. In the similar manner, spraying the lubricant from an end of the die in the widthwise direction enables the lubricant to be applied on both the tapered portion and the parallel portion of the die.
In the present invention, since the forged material extends in the input and output side directions, it is desirable that the parallel portion of the die has a length equal to or above a feed amount at the time of pressing. In addition, if the present invention is used for the steady portion in particular for pressing the front end to the rear end of the hot slab through the steady portion, a slip can be avoided, which is effective.
Further, according to the present invention defined in a preferred twenty-second method embodiment, there is provided a plate thickness pressing method, wherein when forging a hot slab by using a die having a main processing surface consisting of at least an input side tapered portion and a parallel portion, a lubricant is supplied only to the parallel portion of the die to decrease the friction coefficient between the hot slab and the die.
If a forward elongation amount FW is large when subjecting the hot slab 20 to plate thickness pressing, a number of times of pressing is reduced, which is further effective. The forward elongation amount FW largely depends on the friction coefficient between the die 6 and the hot slab 20. Since the lubricant is supplied only to the parallel portion 6a of the die in the present invention, necessary frictional force is generated in the tapered portion 6b, and the forward elongation amount FW is increased without causing a slip in the hot slab 20.